Boris Johnson this week confirmed the country's move to Plan B to curb COVID-19 infections as the Omicron variant continues to spread across England.
The prime minister said at a Downing Street press conference on Wednesday that it is "increasingly clear that Omicron is growing much faster than the previous Delta variant."
He explained that the new measures are designed to curb infections so that the NHS does not fall under "unsustainable pressure" in coming months.
England's chief medical officer Professor Chris Whitty confirmed that the new variant, which was first identified in South Africa, is spreading quickly on top of a high rate of Delta variant transmissions and growing hospitalisations.
There are the seven graphs that explain why the swift spread of Omicron poses a serious risk:
The number of people testing positive for COVID has been on the rise for the past few weeks.
Levels have been consistently high since COVID measures were dropped in the summer, with the UK reporting some of the highest case rates in Europe over the summer months.
This leaves the UK little headroom, and means a surge could quickly push infections to an unsustainable level.
Professor Whitty said: "Omicron is coming on the back of a still high rate of Delta transmission and hospitalisation.
"The Omicron data aren't really going to be visible in this for a few days but they will become more visible over time."
COVID hospitalisations have been a key metric for the government throughout the pandemic, with the second and third national lockdowns brought in to stop the NHS being overwhelmed with coronavirus patients.
Since the vaccine rollout, the link between infections and hospital and admissions has been weakened, meaning surges in infections no longer lead to corresponding spikes in seriously ill patients.
The number of people in hospital with COVID-19 has been drifting down, partially thanks to the vaccine booster programme protecting those who are high risk.
However, Chris Whitty explained that the number of people going into hospital has now "stabilised slightly."
Initial studies suggest the two doses of a vaccine provide far reduced protection against Omicron, posing a risk of a new increase in hospitalisations.
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The number of people dying from COVID is far lower than in the previous waves of COVID, and numbers have been decreasing since the beginning of November.
The seven-day average of COVID deaths per 24 hours was 121, as of 8 December.
"So we still have a significant issue with COVID", Whitty explained.
"I think anybody who speaks to anyone working in the NHS would say that is on top of an incredibly busy system."
The government has relied on the vaccine rollout to stop the NHS from becoming overwhelmed, and Whitty confirmed that the booster programme remains the biggest counter measure against the virus.
Health secretary Sajid Javid defended the move to Plan B to critics by saying it would buy time to get more boosters in arms.
On Sunday Boris Johnson announced that everyone over the age of 18 in England will be able to get a booster by New Years.
Over a third of those eligible for a booster jab have already had theirs.
However, Whitty warned that there are still people in the higher risk groups who have not been boosted yet.
He said: "That is going to become absolutely critical, we think, as we move onto a period where Omicron becomes significant and probably then becomes dominant."
The data shows that the vaccine has been highly effective in reducing the number of people being hospitalised after becoming infected from the previous variants.
The blue bar shows the number of hospitalisations for those who have not been vaccinated, while the yellow bar shows the hospitalisations among those who have received two doses.
Whitty said: "As you can see at every age a really substantial improvement in your protection."
It's not yet clear how much protection the vaccine offers against Omicron specifically, though initial data suggests two doses are significantly less effective against Omicron than against Delta.
The government is monitoring how quickly Omicron is spreading in South Africa in order to get a sense of the risk.
Whitty said: "At this point in time, the big increase in COVID in South Africa is virtually all the Omicron variant and this has now spread all around South Africa."
The graph shows a steep increase in the rate of infection in the past couple of weeks, which has in turn lead to an increase in hospitalisations.
Whitty said there was "around about a 300% increase in hospitalisations over the last week."
South Africa is currently on the red list for England, along with ten other African countries, which means that only British and Irish citizens can return from these countries and must stay in a quarantine hotel upon arrival even if they are fully vaccinated.
The data shows that Omicron cases are increasing fast in the UK.
The two graphs above show the number of cases of COVID-19 which have 'S gene target failure'.
Whitty explained that the S gene target failure is a "marker" for Omicron.
He said: "Virtually all the cases now who've got this marker will have Omicron. A very small number at the bottom will not."
The number of cases with this marker is currently doubling every two to three days.
Whitty explained that we have not yet seen a sharp increase in hospitalisations because at the moment Omicron is mainly spreading among young people who are less likely to be hospitalised for COVID.
However, hospitalisations are likely to go up in coming weeks as Omicron spreads through out the population.
Whitty added: "I'm afraid the data here are now clear.
"We know from previous waves, and this is not particularly surprising, that there is a delay between people becoming infected with COVID and ending up with symptoms and then with hospitalisation.
"In that period, if you're doubling up at the speed we're talking about now, we move from very small numbers to really substantial numbers and it will keep on doubling.
And that really is the reason why these measures have been announced by the prime minister, as agreed by ministers today."
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